1. Technical Field
This application is related to generation of eye-safe optical pulses, and more particularly, to generation of eye-safe optical pulses using the Raman-scattering effect to wavelength shift an infrared pulse.
2. Related Technology
Optical pulses are useful for communication, range finding, imaging, and other purposes. Powerful optical pulses can be safely transmitted through the atmosphere in the 1400-2000 nanometer band without harming human eyes. In this wavelength band, the eye's humor absorbs the optical radiation, preventing damage to the retina. Therefore, optical pulses in this band can be much more intense than similar duration pulses in the visible and near-infrared bands below 1400 nanometers. As a result, active optical devices such as Light Detection and Ranging (LIDAR) systems and range finders can be safely operated more effectively in the 1400-2000 nm band than in shorter visible and near infrared wavelengths. However, laser processes for producing pulses in the 1400-2000 nm band are limited.
Techniques for shifting the wavelength of optical pulses from powerful near-infrared lasers, such as the Nd:YAG lasers, to the 1400-2000 nm band have been sought.
W. Carnuth and T. Trickl, in “A powerful eye-safe infrared aerosol lidar: Application of stimulated Raman backscattering of 1.06 μm radiation”, Rev. Sci. Instrum., Vol. 65, No. 11, pp. 3324-3331, November 1994, discloses using a single backward Raman cell containing deuterium to produce a 1560 nm pulse. The laser pulse is focused into the center of the seed cell. The very short pulses generated with this technique sometimes have damaged optics, and the system appears to have been limited in efficiency to about 20%.
Xu et al., Journal of Modern Optics, Vol. 10., pp 1527-1536, 1999, discloses using a ultraviolet laser pulse at 249 nm with forward scattering from a Raman seed cell to amplify backward scattering in a pump cell, with both cells containing high pressure methane.
Single forward scattering Raman cells with methane have been disclosed in Spinhirne et al., Appl. Optics, Vol. 36, pp. 3475-3490, 1997 and in Mayer and Spuler, Applied Optics, Vol. 43, pp. 3915-3924, 2004. Forward scattering cells filled with methane typically require gas circulating pumps and diode seed lasers operating at the Stokes wavelength.